This invention relates to an apparatus for electrodischarge machining to form holes of desired shapes in a workpiece, by moving a tool electrode vertically down to make a very small gap between the workpiece and the tool electrode and causing repeated electric discharge of over 60V for a long period of time between the workpiece and the tool electrode.
Electrodischarge machining is widely applied to a conductive workpiece, especially for manufacturing molds and sophisticated components. A workpiece to be processed is fixed on a table, and a tool electrode is made of conductive materials such as copper or graphite, and cut into a needed shape and then is held movable in a vertical direction by a quill above the table. Then the tool electrode is moved very close to the workpiece, separated by a very small gap of 10-100 xcexcm. Then if power pulse of over 60V and 1 A is applied between the electrode and the workpiece during the ON time, dielectric liquid of insulation flowing in the gap breaks down and electric discharge (or called electric avalanche) occurs. The electric discharge is a process of energy transmission and energy distribution, producing a very high heat shock to give rise to vaporizing and melting so that the insulation fluid vaporizes and inflates. After electric discharge comes to an end, sudden cooling occurs to let inflated air press inward to form microscopic crater-shaped cavities remained in the surface of the workpiece. The surface roughness of the cavities depends on the pulse-on time and peak current of the power used in the machining process. The quill has to be adjusted in its position for removing debris out of the workpiece, and the gap between the electrode and the workpiece has to be kept proper for continuing the process. Besides, the debris and air bubbles have to be exhausted out at the same time during machining in order to maintain safety.
Flushing operation during the discharging process is effected by the flowing of dielectric fluid caused by pressure difference in the gap between the electrode and a workpiece being machined. There are three kinds methods for removing waste debris:
1. A suckling mouth or nozzle is placed through the electrode or a workpiece, needing the cost for the added component and its route. But it is not easy to obtain balanced smoothness.
2. Flushing oil on the sidewall of the workpiece cavity: this is easy to perform, but pressure of the flushed oil may be not enough to remove the debris in a deep hole, causing secondary discharge in the deep cavity, greatly affecting the precision and surface uniformity of a workpiece.
3. Utilizing a jump motion operation of the electrode: if a low moving speed is used in this method, the electrode may only jump for a short distance, and its speed is very slow, less than 3 m/min, so working time has to be prolonged, wasting much time in waiting, and too frequent jumping may spend too much time to obviously lower efficiency.
It is preferable to use only the third method for improving accuracy of the shape of the cavity in a workpiece, because it can easily acquire balanced flushing pressure. As to accuracy and speed, it is important to design a high-speed movement method and mechanism.
An object of the invention is to provide an electric discharge machining apparatus that can effectively performing vertical movement to a horizontal level, and at the same time to provide a weight balancing device and a positioning device so as to alleviate the whole structure, rigidifying and exposing the structure. Then the apparatus becomes profitable in accommodating, adjusting, completely eliminating necessity of taking apart the device in maintaining and operating.
The invention includes a T-shaped quill movable in the vertical direction to move a tool electrode toward a workpiece while repeatedly producing electric discharge between the workpiece and the tool electrode. The T-shaped quill consists of a square column and a rectangular cubic member provided with two rectangular through holes coaxial with the quill and a T-shaped through hole coaxial with the quill, and the square column has a hole communicating with the T-shaped hole. The invention further includes an electrode mounting device fixed under the square column for fixing a tool electrode, and a gravitation force balancing device for balancing the quill and the electrode mounting device consists of an air cylinder fixed on a support frame moving in parallel to the quill. The air cylinder has a piston rod extending down in the T-shaped through hole and fixed with a level fix plate in the square column. The invention further includes at least two linear motors, which respectively have a yoke fixed in the rectangular through holes of the cubic member and located symmetrical about the balance device and the connect point of the level fix plate in the square column.